A fire extinguisher test in a bank's data centre has gone wrong in an "unprecedented" manner, causing its cash machines, online banking operations and website to go offline.
For much of Saturday, ING's Romanian customers could not access their money.
The bank said the discharge of its gas-based fire suppression system had caused "unexpected" damage to its computer servers.
A report by Motherboard suggests that the equipment was too noisy.
A spokeswoman for ING was unable to confirm this detail.
But she did acknowledge the problem had lasted from 13:00 to 23:00 local time and the bank had been unable to explain the situation to customers as its own communications system had been affected.